1. Field of the Invention:
The present invention relates to a color image forming apparatus suitably applicable to a simplified electrophotographic color image reproducing machine which reproduces a desired color image through a plurality of cycles of an exposure process.
2. Description of the Prior Art:
A color image forming apparatus applied to an electrophotographic image reproducing machine reproduces and copies the color image of a document through a sequential steps of separating the color image into a plurality of separate color images, individually writing color image signals respectively representing the separate color images in an image retainer, and developing and transferring each separate color image, or transferring and fixing all the separate color images after developing all the separate color images.
Such a color image forming apparatus, therefore, the image retainer must be rotated one turn for each of a plurality of separate color image signals to reproduce a color image and the succeeding latent images and the previously developed latent images must be registered. If the write starting position of the succeeding latent image is dislocated from that of the preceding developed latent image, the separate color images are not superposed exactly to deteriorate the print quality. Accordingly, the registration of the separate color images is very important.
Exact registration can be secured by synchronizing the operation of the optical scanning system with the rotation of the image retainer or the transfer drum by using a drum index element as a registration element. In such a case, the optical scanning system is started with reference to drum index signals provided by the drum index element and the subsequent image processing cycles are executed in timed relation with the start of the optical scanning system.
Such a prior art is disclosed in, for example, Japanese Patent Laid-Open Nos. 61-189066 and 61-51178.
Japanese Patent Laid-Open No. 61-189066 discloses a color image reproducing apparatus which reproduces a color image by superposing a plurality of color toner images on an image retainer and by transferring the superposed color toner images to a transfer paper in a single image transfer cycle. In this color image reproducing apparatus, a drum index element is associated with the image retainer to operate the optical scanning system in synchronism with the rotation of the image retainer.
Japanese Patent Laid-Open No. 61-51178 discloses a color image reproducing apparatus which reproduces a color image by forming color toner image individually on the image retainer, by transferring the color toner images individually to a transfer drum to superpose the color toner images on the transfer drum and by transferring the superposed color toner images to a transfer paper. In this color image reproducing apparatus, a drum index element is associated with the transfer drum to operate the optical scanning system in synchronism with the rotation of the transfer drum.
In either known color image reproducing apparatus, a single drum index element is provided and the drum index element provides a drum index signal once every rotation of the associated drum.
FIG. 33 shows the relation between copy start timing and drum index signals in controlling image write operation on the basis of drum index signals, particularly, in copying a color image. Prerotation process, exposure processes respectively for a first color, a second color and a third color, and postrotation process each is executed for every drum index signal.
As obvious from FIG. 33, the postrotation process is ended immediately before, namely, by a fixed time T before, the drum index signal is provided. When the copy start button is operated, the same image copying cycle is repeated.
Accordingly, the color image write operation is started always at a fixed position on the drum. When the color image write operation is started always at a fixed position on the drum, the photosensitive material coating the drum at the fixed position is deteriorated remarkably reducing the life of the same.
Furthermore, when only a single drum index element is provided, the prerotation of the drum is started upon the detection of a first drum index signal, and the start of copying operation for the first color must inevitably wait until the detection of the next drum index signal which is effected after the drum has rotated one full turn, even when 2/3 to 1/2 turn of the drum is sufficient for the prerotation process.
Similarly, the postrotation process requires a little less than a time corresponding to two turns of the drum despite a time corresponding to one and a half turns of the drum is sufficient for the postrotation process, as far as the copying operation is controlled on the basis of the drum index signals which are provided once every one turn of the drum.
Such a necessity of an excessive time for the prerotation process and the postrotation process impedes the reduction of the copying cycle time.
In producing a plurality of copies in such a mode of copying operation, the number of copies produced in a unit time is fixed regardless of the size of copying sheets. That is, as obvious form FIGS. 34A to 34D, the optical scanning time varies with the size of copying sheets, whereas a time required for producing a copy of size B4 is the same as that for producing a copy of size B5, because only one drum index signal is provided every one turn of the drum as shown in FIG. 34 and hence the position of an electrostatic latent image cannot be changed according to the size of the copying sheet. Obviously, it is advantageous to reduce the copying cycle time for producing copies of size B5 as compared with the copying cycle time for producing copies of size B4 so that the number of copies produced in a unit time is increased.